1. Field of the Invention
This invention relates to an image recording apparatus with a maintenance unit, and more particularly to an image recording apparatus with suction means for sucking in ink on the nozzle plate of the inkjet head.
2. Description of the Related Art
Inkjet image recording apparatuses have been widely used. This type of image recording apparatus has an ink head which includes an ink supply source, an ink chamber, and a plurality of nozzles serving as ink discharge outlets. The image recording apparatus shoots ink in the ink chamber from the individual nozzles, thereby recording an image.
The nozzles are arranged in a column on the nozzle plate. The nozzle plate has a water-shedding finish, thereby preventing the adhesion of ink. Even when a water-shedding finish has been given, the adhesion of ink cannot be prevented completely, which may permit ink to collect on the nozzle plate.
Therefore, to maintain the stable ink jet characteristic, the image recording apparatus has to remove the unnecessary ink on the nozzle plate. For this reason, the image recording apparatus has a maintenance unit for removing the unnecessary ink adhering to the nozzle plate. Such an image recording apparatus has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 5-201028.
In this publication, a maintenance unit 1000 as shown in FIG. 23 has been disclosed for a conventional recording apparatus. The image recording apparatus has a head body 1011 in which a plurality of nozzles 1013 are arranged in a column. On the nozzle forming face of the head body 1011, a water-repellent thin film 1014 is provided.
The maintenance unit 1000 includes a vacuum nozzle 1020, a positioning section 1023, an ink trap section 1026, and a vacuum pump 1027. One end of the vacuum nozzle 1020 is supported by the positioning section 1023 which is capable of moving in the direction in which the nozzles 1013 of the ink jet head are arranged. The other end of the vacuum nozzle 1020 is connected to the vacuum pump 1027 via the ink trap section 1026.
The maintenance unit 1000 causes the positioning section 1023 to scan along the arrangement direction of the nozzles 1013, thereby causing the vacuum nozzle 1020 to face the nozzles squarely one after another. At the same time, the maintenance unit 1000 causes the vacuum pump 1027 to operate, thereby making the pressure of one end of the vacuum nozzle 1020 negative. As a result, the maintenance unit 1000 causes the vacuum nozzle 1020 to suck in the unnecessary ink inside and outside each of the nozzles 1013 one after another.
Generally, in the image recording apparatus, a negative pressure is always applied to the ink chamber 1012 as shown in FIG. 24, except when ink is discharged. In FIG. 24, ink is indicated by reference symbol i. With the negative pressure, the ink in the nozzle 1013 forms a concave meniscus in the nozzle 1013.
The negative pressure is generally set so as to form a meniscus in a specific position in the nozzle. More specifically, the negative pressure is so set that the position where the surface tension of the meniscus balances with the negative pressure comes to the specific position in the nozzle. Since the meniscus is formed by the negative pressure, it is not desirable that ink should be sucked out of the nozzle carelessly.
However, when suction is applied under negative pressure, the ink i in the nozzle 1013 and ink chamber 1012 is sucked into the vacuum nozzle 1020 by suction exceeding the negative pressure in the ink chamber 1012, as shown in FIG. 25. Specifically, the maintenance unit 1000 sucks in ink in such a manner that it draws out ink forcibly from the inner part of the nozzle 1013. As a result, the fluid level of the ink i retreats from the nozzle 1013 into the ink chamber 1012. As described above, since the negative pressure in the ink chamber is set so as to form a meniscus in the nozzle, the fluid level of ink in the ink chamber 1012 moves again so as to go back into the nozzle 1013, even during suction by the vacuum nozzle 1020. When ink is sucked in under negative pressure as described above, the fluid level of ink moves unstably between the nozzle 1013 and the interior of the ink chamber 1012. As a result of the movement, the ink i pulls air into the ink chamber 1012.
When air has been pulled into the ink chamber 1012, there is a danger that the air will remain in the ink chamber 1012 in air bubbles. Since air bubbles cause pressure loss in the ink chamber 1012, there is a possibility that the inkjet head will lose the desired ink shooting characteristic.